A steel for a welded structure excellent in strength and weldability is used for a steel oil tank, such as an oil tank of a crude oil carrier or an aboveground or underground crude oil tank, for transporting or storing crude oil. The problems to be solved in relation to corrosion damage of a crude oil tank have been: 1) to alleviate corrosion of steel plates, especially to alleviate local corrosion damage in the form of pitting that progresses at a comparatively high rate; and 2) to reduce the amount of solid sulfur that precipitates on the surfaces of steel plates in a gas phase and causes sludge to form. These problems are outlined below.
1) Alleviation of Corrosion of Steel Plates
The inside of a crude oil tank is exposed to a corrosive environment caused by water, salts and corrosive gas components contained in crude oil (cf. Recommended Practice of Corrosion Control and Protection in Aboveground Oil Storage Tank HPIS G, p. 18 (1989-90), published by the High Pressure Institute of Japan, and SR242—Study on Cargo Oil Tank Corrosion of Oil Tanker, Outline of Research Activities in Fiscal Year 2000 of the Shipbuilding Research Association of Japan). A peculiar corrosive environment forms especially on the inside of an oil tank of a crude oil carrier because of elements such as volatile components of crude oil, contaminating seawater, salts in oil field brine, the marine engine exhaust gas called inert gas that is introduced to the tank for preventing explosions, and water condensation caused by the temperature fluctuation between daytime and nighttime. In such an environment, a steel is damaged by general corrosion and local corrosion in the form of pitting.
As a result, corrosion cavities roughly 10 to 30 mm in diameter form in quantities on the floor plate of an oil tank of a crude oil carrier, and the corrosion cavities progress at a rate of 2 to 3 mm per year. This is far greater than the average rate of thickness loss caused by corrosion, 0.1 mm per year, which is taken into consideration in the design of a hull. The local corrosion of structural members of a crude oil tank is particularly detrimental, because when corrosion progresses locally, loads on the corroded portions increase beyond what is expected in the design, and large strain and/or plastic deformation occur(s), leading to possible destruction of the whole structure. Thus, countermeasures against local corrosion are indispensable. In addition, it is difficult to predict the location of local corrosion and its rate of progress. For these reasons, development of a steel for a welded structure excellent in strength and weldability and, at the same time, having good corrosion resistance especially capable of decreasing the progress rate of local corrosion has been in demand.
2) Reduction of the Amount of Solid Sulfur that Precipitates on the Surfaces of Steel Plates in a Gas Phase and Causes Sludge to Form
In addition to the corrosion damage mentioned above, a large quantity of solid sulfur forms and precipitates on the internal surface of a steel oil tank, especially on the reverse side surface of a steel plate of an upper deck (deck plate). This is because SO2 and H2S in a gas phase react and form solid sulfur, with the iron rust on a corroded steel plate surface acting as a catalyst. The formation of fresh rust resulting from the corrosion of a steel plate and the precipitation of solid sulfur take place alternately and, as a result, a multi-layered corrosion product composed of iron rust and solid sulfur forms. Since a solid sulfur layer is brittle, the corrosion product composed of iron rust and solid sulfur easily exfoliates, falls off and accumulates as sludge at the bottom of an oil tank. The amount of sludge collected from a very large crude oil carrier during a periodical inspection is reported to amount to 300 tons or more, and for that reason, reduction of the amount of sludge composed mainly of solid sulfur has been required from the viewpoint of maintenance.
Corrosion prevention by painting and lining has generally been employed as a technique for protecting a steel material against corrosion and simultaneously decreasing sludge composed mainly of solid sulfur, and corrosion prevention by spraying zinc and/or aluminum has also been proposed (cf. Recommended Practice of Corrosion Control and Protection in Aboveground Oil Storage Tank HPIS G, p. 18 (1989-90), published by the High Pressure Institute of Japan). However, in addition to the economical problems of the time and costs involved in re-painting the reverse side of all the deck plates of a very large crude oil carrier, there has also been a technical problem in that protection by painting and/or lining also requires periodical inspections and repair, because corrosion inevitably progresses as a result of microscopic defects caused during the application of protective layers and age-related degradation.
Despite the above, no technology to suppress the precipitation of solid sulfur on a steel plate surface by improving the corrosion resistance of the steel plate itself in a crude oil tank environment has been disclosed. In this situation, in the field of a steel for a welded structure such as an oil tank, development of a steel for a welded structure excellent in corrosion resistance and capable of suppressing the formation of sludge mainly composed of solid sulfur has been in demand from the viewpoints of enhancing the reliability and extending the service life of the structure.
Here, an overview is given below regarding technologies so far proposed to solve the problems 1) and 2) above, peripheral technologies proposed in relation thereto and problems involved in the proposed technologies.
1) Measures to Alleviate Corrosion of Steel Plates and Problems of Conventional Technologies
Technologies so far proposed to alleviate corrosion, especially local corrosion, of a steel plate occurring on the inside of a crude oil tank are described below. Ordinary steels for welded structures have generally been used without protective coating for a crude oil tank, either that of a crude oil carrier or that constructed aboveground or underground. Painting has conventionally been the most commonly employed corrosion prevention method, and protective painting with an epoxy resin and/or zinc rich primer, heavy-duty coating with an epoxy resin mixed with glass flakes and the like have been proposed for this purpose. Besides these, hot dip galvanized steels with paint coating have been used for handrails and piping of an oil carrier in view of its excellent corrosion resistance in an environment where the steels are alternately exposed to seawater and crude oil. In addition to the above, the following technologies have been proposed to provide corrosion-resistant steels having better corrosion resistance than ordinary steels do and being suitable for use in the interior of a crude oil tank.
Japanese Unexamined Patent Publication No. S50-158515 proposes a Cu—Cr—Mo—Sb steel as a steel for an oil loading pipe on the basis that the steel exhibits excellent corrosion resistance in an environment where a steel, such as an oil loading pipe, is exposed to crude oil and seawater alternately or simultaneously. The corrosion-resistant steel disclosed in the publication contains 0.2 to 0.5% Cr as a main component and, in addition, 0.1 to 0.5% Cu, 0.02 to 0.5% Mo, and 0.01 to 0.1% Sb.
Japanese Unexamined Patent Publication No. 2000-17381 proposes a Cu—Mg steel as a corrosion-resistant steel for shipbuilding on the basis that the steel exhibits excellent corrosion resistance in an environment where a steel is used for a hull outer plate, a ballast tank, an oil tank (crude oil tank) of a crude oil carrier, or a cargo hold of an ore/coal carrier. The corrosion-resistant steel disclosed in the publication contains 0.01 to 2.0% Cu and 0.0002 to 0.0150% Mg as main components and, in addition, 0.01 to 0.25% C, 0.05 to 0.50% Si, 0.05 to 2.0% Mn, 0.10% or less P, 0.001 to 0.10% S, and 0.005 to 0.10% Al.
Japanese Unexamined Patent Publication No. 2001-107179 proposes a high-P—Cu—Ni—Cr-high-Al steel as a corrosion-resistant steel for an oil loading tank on the basis that the steel exhibits excellent corrosion resistance at the reverse side of the deck plate of an oil loading tank and low welding crack sensitivity. The corrosion-resistant steel disclosed in the publication contains 0.04 to 0.1% P, 0.005% or less S, 0.1 to 0.4% Cu, 0.05 to 0.4% Ni, 0.3 to 4% Cr and 0.2 to 0.8% Al as main components and, in addition, 0.12% or less C, 1.5% or less Si and 0.2 to 3% Mn, and satisfies the expression Pcm≦0.22, where Pcm=[% C]+[% Si]/30+[% Mn]/20+[% Cu]/20+[% Ni]/60+[% Cr]/20+[% Mo]/15+[% V]/10+5[% B].
Japanese Unexamined Patent Publication No. 2001-107180 proposes a low-P—Cu—Ni—Cr-high-Al steel as a corrosion-resistant steel for an oil loading tank on the basis that the steel exhibits excellent corrosion resistance at the reverse side of the deck plate of an oil loading tank, as well as being excellent in a balance between mechanical properties and weldability at large-heat-input welding exceeding 100 kJ. The corrosion-resistant steel disclosed in the publication contains 0.035% or less P, 0.005% or less S, 0.1 to 0.4% Cu, 0.05 to 0.4% Ni, 0.3 to 4% Cr and 0.2 to 0.8% Al as main components and, in addition, 0.12% or less C, 1.5% or less Si and 0.2 to 3% Mn, and satisfies the expression Pcm≦0.22, where Pcm=[% C]+[% Si]/30+[% Mn]/20+[% Cu]/20+[% Ni]/60+[% Cr]/20+[% Mo]/15+[% V]/10+5[% B].
Japanese Unexamined Patent Publication No. 2002-12940 proposes a Cu contained steel, a Cr contained steel and an Ni contained steel as corrosion-resistant steels for oil loading tanks and methods for producing the same on the basis that each of the steels exhibits: excellent corrosion resistance, more specifically, such good durability as to minimize the progress of rust under a primer coating film and thus to extend the service life of the coating film after the application of the primer coating in a corrosive atmosphere at the upper part of an oil loading tank, i.e. in an acid-dew-point corrosive environment caused by corrosive components included in the engine exhaust gas that is introduced into an oil loading tank; and the feature of excellent weldability. Each of the corrosion-resistant steels disclosed in the publication: is used on condition that primer coating is applied; contains one or more of 0.1 to 1.4% Cu, 0.2 to 4% Cr and 0.05 to 0.7% Ni as basic component(s) and, in addition, 0.16% or less C, 1.5% or less Si, 3.0% or less Mn, 0.035% or less P and 0.01% or less S; and satisfies the expression Pcm≦0.22, where Pcm=[% C]+[% Si]/30+[% Mn]/20+[% Cu]/20+[% Ni]/60+[% Cr]/20+[% Mo]/15+[% V]/10+5[% B].
Japanese Unexamined Patent Publication No. 2003-105467 proposes a Cu—Ni steel as a corrosion-resistant steel for an oil loading tank excellent in corrosion resistance at a weld on the basis that the steel exhibits excellent corrosion resistance both at base material after application of primer coating and at a weld to which primer coating is not applied and makes it possible to use an existing welding wire for a carbon steel. The corrosion-resistant steel disclosed in the publication: is used on condition that primer coating is applied; contains 0.15 to 1.4% Cu as a basic component and, in addition, 0.16% or less C, 1.5% or less Si, 2.0% or less Mn, 0.05% or less P and 0.01% or less S; and satisfies the expression Pcm≦0.24, where Pcm=C+Si/30+Mn/20+Cr/20+Cu/20+Ni/60+Mo/15+V/10+5B.
Japanese Unexamined Patent Publication No. 2001-214236 proposes a Cu contained steel, a Cr contained steel, an Mo contained steel, an Ni contained steel, an Sb contained steel, and an Sn contained steel as corrosion-resistant steels for crude oil or heavy oil storage tanks on the basis that each of the steels exhibits excellent corrosion resistance when it is used for a crude oil carrier, an oil tank or the like for storing a liquid fuel or a raw fuel such as crude oil or heavy oil. Each of the corrosion-resistant steels disclosed in the publication contains one or more of 0.01 to 2.0% Cu, 0.01 to 7.0% Ni, 0.01 to 10.0% Cr, 0.01 to 4.0% Mo, 0.01 to 0.3% Sb and 0.01 to 0.3% Sn as basic component(s) and, in addition, 0.003 to 0.30% C, 2.0% or less Si, 2.0% or less Mn, 0.10% or less Al, 0.050% or less P and 0.050% or less S.
Japanese Unexamined Patent Publication No. 2002-173736 proposes a Cu—Ni—Cr steel as a corrosion-resistant steel for a tank for transporting or storing crude oil on the basis that the steel exhibits excellent corrosion resistance. The corrosion-resistant steel disclosed in the publication contains 0.5 to 1.5% Cu, 0.5 to 3.0% Ni and 0.5 to 2.0% Cr as basic components and, in addition, 0.001 to 0.20% C, 0.10 to 0.40% Si, 0.50 to 2.0% Mn, 0.020% or less P, 0.010% or less S and 0.01 to 0.10% Al.
Japanese Unexamined Patent Publication No. 2003-82435 proposes an Ni contained steel and a Cu—Ni steel as steel materials for cargo oil tanks on the basis that each of the steels exhibits excellent corrosion resistance, more specifically, excellent resistance to general corrosion in an environment containing inert gas where wet and dry are repeated alternately. Each of the corrosion-resistant steels disclosed in the publication contains 0.05 to 3% Ni as a basic component and, in addition, 0.01 to 0.3% C, 0.02 to 1% Si, 0.05 to 2% Mn, 0.05% or less P, 0.01% or less S and, as required, one or more of Mo, Cu, W, Ca, Ti, Nb, V, B, Sb, and Sn.
In addition to the above, the following technologies have been proposed regarding corrosion resistant steels for a ballast tank of a marine vessel, although the steels are not for crude oil tank use.
Japanese Examined Patent Publication No. S49-27709 proposes a Cu—W steel and a Cu—W—Mo steel as corrosion-resistant low-alloy steels on the basis that each of the steels exhibits excellent corrosion resistance when used for a ballast tank. Each of the corrosion-resistant steels disclosed in the publication contains 0.15 to 0.50% Cu and 0.05 to 0.5% W as basic components and, in addition, 0.2% or less C, 1.0% or less Si, 1.5% or less Mn and 0.1% or less P and, as required, 0.05 to 1.0% Mo.
Japanese Unexamined Patent Publication No. S48-509217 proposes, in patent document 11, a Cu—W steel and a Cu—W—Mo steel as corrosion-resistant low-alloy steels on the basis that each of the steels exhibits excellent corrosion resistance when used for a ballast tank. Each of the corrosion-resistant steels disclosed in the publication contains 0.15 to 0.50% Cu and 0.01 to less than 0.05% W as basic components and, in addition, 0.2% or less C, 1.0% or less Si, 1.5% or less Mn and 0.1% or less P and, as required, 0.05 to 1.0% Mo.
Japanese Unexamined Patent Publication No. S48-50922 proposes a steel containing Cu, W and one or more of Ge, Sn, Pb, As, Sb, Bi, Te and Be as a corrosion-resistant low-alloy steel on the basis that the steel exhibits excellent corrosion resistance, more specifically excellent resistance to local corrosion in a ballast tank. The corrosion-resistant steel disclosed in the publication contains 0.15 to 0.50% Cu, 0.05 to 0.5% W and one or more of Ge, Sn, Pb, As, Sb, Bi, Te and Be by a total of 0.01 to 0.2% as basic components and, in addition, 0.2% or less C, 1.0% or less Si, 1.5% or less Mn and 0.1% or less P and, as required, 0.01 to 1.0% Mo.
Japanese Unexamined Patent Publication No. S49-3808 proposes a Cu—Mo steel as a corrosion-resistant low-alloy steel on the basis that the steel exhibits excellent corrosion resistance in a ballast tank, high strength and good weldability. The corrosion-resistant steel disclosed in the publication contains 0.05 to 0.5% Cu and 0.01 to 1% Mo as basic components and, in addition, 0.2% or less C, 1.0% or less Si, 0.3 to 3.0% Mn and 0.1% or less P.
Japanese Unexamined Patent Publication No. S49-52117 proposes a Cr—Al steel as a seawater corrosion-resistant low-alloy steel on the basis that the steel is excellent in corrosion resistance in seawater, more specifically in resistance to pitting corrosion and crevice corrosion, which are likely to occur in quantity to a steel containing alloying elements. The corrosion-resistant steel disclosed in the publication contains 1 to 6% Cr and 0.1 to 8% Al as basic components and, in addition, 0.08% or less C, 0.75% or less Si, 1% or less Mn, 0.09% or less P and 0.09% or less S.
Japanese Unexamined Patent Publication No. H7-310141 proposes a Cr—Ti steel as a seawater corrosion-resistant steel for use in a high-temperature and high-humidity environment and a method for producing the same on the basis that the steel exhibits excellent resistance to seawater corrosion in a high-temperature and high-humidity environment of a marine vessel, namely in a ballast tank or in a seawater pipe and excellent toughness at a heat-affected zone (HAZ). The corrosion-resistant steel disclosed in the publication contains 0.50 to 3.50% Cr as a basic component and, in addition, 0.1% or less C, 0.50% or less Si, 1.50% or less Mn and 0.005 to 0.050% Al.
Japanese Unexamined Patent Publication No. H8-246048 proposes a Cr contained steel in a method for producing a seawater corrosion-resistant steel excellent in toughness of a HAZ for use in a high-temperature and high-humidity environment on the basis that the steel exhibits excellent resistance to seawater corrosion in a high-temperature and high-humidity environment of a marine vessel, namely in a ballast tank or a seawater pipe. The corrosion-resistant steel disclosed in the publication contains 1.0 to 3.0% Cr and 0.005 to 0.03% Ti as basic components and, in addition, 0.1% or less C, 0.10 to 0.80% Si, 1.50% or less Mn and 0.005 to 0.050% Al.
Here, problems of the conventional technologies described above are explained.
The problems arising when corrosion is mitigated by means of corrosion prevention coating such as primer coating, heavy-duty coating or metal spraying have been that: the application work entails substantial costs; and, in addition, corrosion develops to a extent comparable to a case of bare use in 5 to 10 years of normal use at the longest, because local corrosion inevitably occurs and propagates from microscopic defects in protective coating layers caused during the application work and other defects resulting from age-related degradation. Another problem has been that periodical inspections and repair are indispensable and maintenance costs are involved as a consequence. Yet another problem has been that, with regard to local corrosion at the floor plate of an oil tank, the rate of progress of local corrosion occurring after protective coating layers have been degraded is substantially the same as that occurring in bare use.
The problems of the steel for an oil loading pipe disclosed in Japanese Unexamined Patent Publication No. S50-158515 have been that: since it contains Cr, which is detrimental to corrosion resistance in a crude oil tank environment, in excess of 0.1%, the rate of progress of local corrosion at the floor plate of an oil tank is not reduced and the cost effect of corrosion resistance is insufficient to justify the total addition amount of the alloying elements; and the weldability of the steel is poor in comparison with an ordinary steel because it contains Cr.
The problems of the corrosion-resistant steel for shipbuilding disclosed in Japanese Unexamined Patent Publication No. 2000-17381 have been that: since it contains Mg as an indispensable element, the production of the steel is unstable; and, according to the studies by the present inventors, the rate of progress of local corrosion at the floor plate of an oil tank is not reduced by the use of a Cu—Mg steel and the cost effect of corrosion resistance is insufficient to justify the total addition amount of the alloying elements.
The problems of the corrosion-resistant steel for an oil loading tank (a high-P—Cu—Ni—Cr-high-Al steel) disclosed in Japanese Unexamined Patent Publication No. 2001-107179 have been that: since it contains Cr, which is detrimental to corrosion resistance in an environment of a crude oil tank floor plate, by 0.3 to 4% in excess of 0.1%, the rate of progress of local corrosion at the floor plate of an oil tank is not reduced and the cost effect of corrosion resistance is insufficient to justify the total addition amount of the alloying elements; and the weldability of the steel is poor in comparison with an ordinary steel because it contains Cr.
The problems of the corrosion-resistant steel for an oil loading tank (a low-P—Cu—Ni—Cr-high-Al steel) disclosed in Japanese Unexamined Patent Publication No. 2001-107180 have been that: since it contains Cr, which is detrimental to corrosion resistance in an environment of a crude oil tank floor plate, by 0.3 to 4% in excess of 0.1%, the rate of progress of local corrosion at the floor plate of an oil tank is not reduced and the cost effect of corrosion resistance is insufficient to justify the total addition amount of the alloying elements; the weldability of the steel is poor in comparison with an ordinary steel because it contains Cr; and, although the publication maintains that the steel after application of a primer coating suppresses corrosion under a coating film in a gas phase to which the reverse side of a deck plate or the like of an oil tank is exposed, since the steel contains comparatively large amounts of Cr and Al, the rate of corrosion propagating in the thickness direction from defects in a coating film is not reduced despite the width of blisters occurring from defects in the coating film being reduced.
The problem of the corrosion-resistant steels (Cu—Ni steels) for oil loading tanks disclosed in Japanese Unexamined Patent Publication Nos. 2002-12940 and 2003-105467 has been that, though the publications maintain that Cu and Ni are effective in enhancing corrosion resistance, more specifically resistance to corrosion under a coating film, and Mo is detrimental to corrosion resistance but is effective for enhancing strength, since any of the Cu—Ni—Mo steels proposed as corrosion-resistant steels in the example contains Mo in excess of the upper limit (0.2%) of the present invention, the effect of suppressing the progress of local corrosion at the floor plate of a crude oil tank is not achieved.
The problems of the corrosion-resistant steels (a Cu contained steel, a Cr contained steel, an Mo contained steel, an Ni contained steel, an Sb-contained steel and an Sn-contained steel) for crude oil or heavy oil storage tanks disclosed in Japanese Unexamined Patent Publication No. 2001-214236 have been that: large amounts of alloying elements must be added in order to obtain excellent corrosion resistance as the example shows that it is indispensable to add one or more of 0.22 to 1.2% Cu, 0.3 to 5.6% Cr, 0.5 to 6.2% Ni, 0.25 to 7.56% Mo, 0.07 to 0.25% Sb and 0.07 to 1.5% Sn; and thus the economical efficiency and weldability of the proposed steels are poor.
The problems of the corrosion-resistant steel for a tank for transporting or storing crude oil (a Cu—Ni—Cr steel) disclosed in Japanese Unexamined Patent Publication No. 2002-173736 have been that: the steel contains 0.5 to 1.5% Cu, 0.5 to 3.0% Ni and 0.5 to 2.0% Cr as basic components, thus large amounts of alloying elements must be added for the effect to appear; thus the economical efficiency and weldability of the proposed steels are poor; and further, since the steel contains Cr, which is detrimental to corrosion resistance in an environment of a crude oil tank floor plate, in excess of 0.1%, the rate of progress of local corrosion at the floor plate of an oil tank is not reduced and the cost effect of corrosion resistance is insufficient to justify the total addition amount of the alloying elements.
With regard to the steels for cargo oil tanks (Ni contained steels and Cu—Ni steels) disclosed in Japanese Unexamined Patent Publication No. 2003-82435, steel components are studied which decrease the progress of local corrosion in an experimental corrosive environment simulating not that at the floor plate of an oil tank, but at the reverse side of a deck plate. Table 4 of the publication lists the following as the steels that contain Cu, Ni and Mo as basic components but not Cr: sample nos. B4 (0.43% Cu-0.18% Ni-0.26% Mo), B6 (0.33% Cu-0.31% Ni-0.35% Mo), B13 (0.38% Cu-0.12% Ni-0.44% Mo), B15 (0.35% Cu-0.28% Ni-0.31% Mo), B19 (0.59% Cu-0.16% Ni-0.22% Mo) and B20 (0.59% Cu-0.44% Ni-0.22% Mo). The problems of the steels have been that: all of these steels requires relatively large addition amounts of alloying components even though only the basic components are taken into consideration and results in unfavorable costs and weldability; and further, in order to realize excellent corrosion resistance in an environment of a crude oil tank floor plate, it is necessary to use an Ni-contained steel or a Cu—Ni steel, control the number of inclusions larger than 30 μm in grain size to less than 30/cm2, and control the pearlite ratio Ap in the metallographic structure and the carbon content in the steel so as to satisfy the expression Ap/C≦130.
Next, the problems of the corrosion-resistant steels proposed for the use in the ballast tank of a marine vessel are explained.
The problems of the corrosion-resistant low-alloy steels (a Cu—W steel and a Cu—W—Mo steel) disclosed in Japanese Examined Patent Publication No. S49-27709 have been that: since the steels do not contain Al according to the chemical compositions of the invention steels shown in Table 1 of the examples described in patent document 10, resistance to local corrosion is not secured in the case of the floor plate of a crude oil tank; and further the proposed steel, which is not Al-killed steels, is hardly applicable to the latest shipbuilding use from the viewpoints of the cleanliness of the steels and the toughness of welds.
The problems of the corrosion-resistant low-alloy steels (a Cu—W steel and a Cu—W—Mo steel) disclosed in Japanese Unexamined Patent Publication No. S48-50921 have been that: since the steels do not contain Al according to the chemical compositions of the invention steels shown in Table 1 of the examples described in the patent, resistance to local corrosion is not secured in the case of the floor plate of a crude oil tank; and further the proposed steel, which is obviously not Al-killed steels, is hardly applicable to the latest shipbuilding use from the viewpoints of the cleanliness of the steels and the toughness of welds.
The problems of the corrosion-resistant low-alloy steel disclosed in Japanese Unexamined Patent Publication No. S48-50922 have been that: since the steel contains 0.15 to 0.50% Cu, 0.05 to 0.5% W and further one or more of Ge, Sn, Pb, As, Sb, Bi, Te and Be by 0.01 to 0.2%, the proposed steel is markedly poor in hot workability; since the steel does not contain Al according to the chemical compositions shown in Table 1 of the patent, local corrosion resistance is not secured in the case of a floor plate of a crude oil tank; and further the proposed steel, which is obviously not an Al-killed steel, is hardly applicable to the latest shipbuilding use from the viewpoints of the cleanliness of the steel and the toughness of a weld.
The problems of the Cu—Mo steel proposed in Japanese Unexamined Patent Publication No. S49-3808 as a corrosion-resistant low-alloy steel for ballast tank use is that: since the steel is obviously required to contain not less than 0.008% S in order to obtain desired corrosion resistance in a ballast tank environment according to the chemical composition of the proposed steel shown in the examples described in the patent, the proposed steel cannot secure local corrosion resistance comparable with that of a steel according to the present invention in the case of a crude oil tank floor plate; since the steel does not contain Al, local corrosion resistance is not secured in the case of a floor plate of a crude oil tank; and further the proposed steel, which is obviously not an Al-killed steel, is hardly applicable to the latest shipbuilding use from the viewpoints of the cleanliness of the steel and the toughness of a weld.
The problem of the corrosion-resistant steels disclosed in Japanese Unexamined Patent Publication Nos. S49-52117, H7-310141 and H8-246048 has been that each of the steels contains not less than 0.5% Cr as a basic component and cannot secure local corrosion resistance in the case of the floor plate of a crude oil tank.
Other than the conventional technologies mentioned above, some technologies regarding low-alloy corrosion-resistant steels for other applications have been disclosed. Some comments are given thereon hereafter.
Automobile undercarriage members suffer wet corrosion involving chloride ions with deicing salt attaching thereto. With regard to low-alloy steels for automobile undercarriage members excellent in pitting corrosion resistance that cope with such corrosion problem, there are, for instance: the technology characterized by adding Cu, Ni, Ti and P to a steel and, by so doing, forming a protective film composed of phosphate on the surface of the steel (such as the one disclosed in Japanese Unexamined Patent Publication No. S62-243738); and the technology characterized by adding P and/or Cu to a steel and, by so doing, making the formed rust layer amorphous and dense so as to enhance the protective capability of the rust layer (such as the one disclosed in Japanese Unexamined Patent Publication No. H2-22416). In addition, many steelmakers have developed and commercialized seawater-resistant low-alloy steels having improved seawater resistance (cf. “Corrosion-resistant Low-alloy Steel” by Iwao Matsushima, p. 117, published from Chijin Shokan in 1995).
In the case of those steels for automobile undercarriage parts excellent in pitting corrosion resistance and other weatherproof steels, although it is true that a protective dense rust layer forms on the surface even when such a steel is used in a salt damage environment, such excellent pitting corrosion resistance is obtained only in an environment where wet and dry are repeated properly and resultantly a protective dense rust layer forms spontaneously and not in an environment where the steel surface is always wet. Thus, such excellent pitting corrosion resistance is not obtained in an environment where the time of wetting is long or the steel surface is always wet. On the other hand, in the case of the seawater-resistant low-alloy steels mentioned above, though they often exhibit better performance than ordinary steels regarding the kind of corrosion resistance to be evaluated in terms of an average thickness loss rate, they are not viewed as distinctly superior to ordinary steels regarding a local corrosion rate of progress (cf. “Corrosion-resistant Low-alloy Steel” by Iwao Matsushima, p. 112, published from Chijin Shokan in 1995).
As has hitherto been explained, in the application of a steel to a welded structure such as a crude oil tank, development of a low-alloy steel having a low local corrosion progress rate even though general corrosion may occur has been looked for from the viewpoints of enhancing the reliability of a structure and extending the service life. As for the technologies for decreasing the progress of local corrosion at the floor plate of a crude oil tank, merely the methods of applying a protective lining to the floor plate have been proposed. There have been a number of proposals regarding corrosion-resistant steels to mitigate the corrosion occurring in the environment of a ballast tank, which is similar to the environment of a crude oil tank intended in the present invention, or in the environment at the reverse side of a deck plate of a crude oil tank. However, there has been only one proposal regarding a corrosion-resistant steel having a low local corrosion progress rate at the floor plate of a crude oil tank, which is the invention disclosed in the Japanese Unexamined Patent Publication No. 2003-82435 mentioned earlier.
2) Measures to Reduce the Amount of Solid Sulfur that Precipitates on the Surfaces of Steel Plates in a Gas Phase and Causes Sludge to Form and Problems of Conventional Technologies
Corrosion prevention by painting and lining has commonly been employed as a technique to protect steel from corrosion and, at the same time, reduce sludge composed mainly of solid sulfur. Corrosion prevention by spraying zinc and/or aluminum has also been proposed (cf. Recommended Practice of Corrosion Control and Protection in Aboveground Oil Storage Tank HPIS G, p. 18 (1989-90), of the High Pressure Institute of Japan). However, like in the case of corrosion reduction measures, the problems of the technologies have been that: the application work entails economic costs; and, in addition, since corrosion inevitably progresses as a result of microscopic defects in protective layers caused during the application work and age-related degradation, periodical inspections and repair are indispensable and the service life is limited to 5 to 10 years, even when painting and lining are applied.
Despite the above problems, there has been disclosed no technology to decrease the precipitation of solid sulfur on a steel surface by improving the corrosion resistance of steel itself in a crude oil tank environment. In such a situation, in the application of a steel to a welded structure such as an oil tank, development of a steel for a welded structure excellent in corrosion resistance and capable of decreasing the formation of sludge mainly composed of solid sulfur has been looked for from the viewpoints of enhancing the reliability of the structure and extending the service life.